The sharpening of atomic energy levels with decreasing temperature is the motivation for using atoms cooled to just a fraction above absolute zero as the detection medium for metrological applications. Ultracold atom “fountains” already have been used in atomic clocks and in gravimeters and gravity gradiometers. However, the cooling process provides a thermal cloud of warmer atoms that surround the cold-atom core, and as detection of the atoms usually relies on monitoring fluorescence or absorption transitions between energy levels, these thermal atoms broaden the spectra and reduce sensitivity.

McGuirk et al. have used a balanced detection system in which the cold-atom cloud is separated into two equal populations and the differential fluorescence from atom states in each population cancels, thus reducing technical noise sources such as those associated with fluctuations in the probe laser amplitude and frequency. They also used a modulation transfer technique that selects only those atoms that are stationary, which would differentiate the cold atoms from the surrounding thermal cloud. This approach could lead to an improvement in the signal-to-noise ratio by nearly one order of magnitude. — ISO